The present invention relates in general to the field of resistive memory elements, and more particularly to resistive memory elements based on doped amorphous carbon, for example resistive random-access memory elements.
Resistive switching refers to a physical phenomenon occurring in a material that suddenly changes its resistance under action of a strong current or electric field. The change is non-volatile and reversible. Several classes of switching materials (ranging from metal oxides to chalcogenides) have been proposed in the past. The performances of these materials are appreciated mainly in terms of power consumption, integration density potential, retention, and endurance. Typical resistive switching systems are capacitor like devices, where electrodes are ordinary metals and the dielectric a resistive switching material, e.g., a transition metal oxide.
An interesting application of resistive switching is the fabrication of non-volatile resistive random-access memories (RRAM), which are promising candidates to replace conventional flash memories as they offer better scalability, higher integration density, lower cost, and lower power consumption. Recently, amorphous carbon (a-C) has been proposed as a resistive switching material for RRAM applications. Compared to oxide-based RRAM, carbon promises higher memory density and lower power consumption. The mono-atomic nature of carbon would make a carbon-based memory cell scalable even to single bonds. Such cell dimensions would limit the reset current, thus reducing the power consumption. In addition, the high resilience of carbon would enable operation at high temperatures.
Another intrinsic advantage of a-C-based RRAM is the switching mechanism. Amorphous carbon is mainly formed by sp2 bonds (conductive) and sp3 bonds (insulating). When a set voltage is applied across the a-C layer, the electric field and the Joule heating induce a clustering of sp2 bonds, bringing the cell into a low resistive state. When another voltage (reset) is applied across the cell, causing a high current to flow through the sp2 filaments, these filaments break down owing to Joule heating, and the cell returns to a high resistance state. No electrochemical reaction is involved: the resistive switching in carbon is unipolar, i.e., the memory can be set and reset by means of voltages of the same polarity. In contrast, resistive switching in oxide-based RRAM occurs owing to the reduction (set) and oxidation (reset) of oxygen vacancies. Therefore, voltages of opposite polarity are needed to set and reset the cell (bipolar switching). Unipolar resistive switching, as it occurs in carbon-based RRAM, simplifies the circuit design of the memory devices, compared to bipolar switching circuits.
Another advantage of carbon-based RRAM is that no “conditioning step” is required. This step is needed in oxide-based RRAM and involves the application of a high voltage across the cell to induce a soft breakdown and form the channel in which the filaments will then grow. Because the conditioning voltage is typically much higher than the set voltage, this step might degrade the device endurance and therefore is not desirable.
To be suitable for RRAM applications, a-C needs to have the right trade-off in the content between sp2 (conductive) and sp3 (insulating) bonds. As one increases the sp3 content, the resistance of the film and the set voltage increase accordingly. This happens because the electric field that is needed to induce the clustering of sp2 bonds is higher. On the other hand, during the reset process, the sp2 filament is surrounded by a high number of sp3 bonds. This guarantees that the reset current flows only along that filament and makes it easier to break the filament down by Joule heating. In general, high resistance (high sp3 content) a-C is needed to provide reversible resistive switching.
The production of high-resistance a-C is not trivial because it requires either special deposition tools or the introduction of certain dopants in the carbon layer. Regarding the first option, it is possible to deposit tetrahedral amorphous carbon (ta-C), which presents a high sp3 content (>70%), by using deposition techniques such as filtered cathodic arc deposition (FCAD) or pulsed laser deposition (PLD).
Another way to increase the electrical resistance of a-C is to dope it with H2, to form sp3 bonds with carbon. It was for instance demonstrated that the resistance of hydrogenated amorphous carbon (a-C:H) could be tuned by seven orders of magnitude by adjusting the H2 content and that the resistive switching in a-C:H is reversible.
Some manufacturers have recently focused their research efforts on developing new carbon-based resistive memories. To resolve the issue of producing high-sp3-content carbon layers, they proposed different approaches, namely to:                use metal electrodes with a steering and a compressive function to increase and stabilize sp3 bonds, also compensating the H2 outgassing in a-C:H;        thermally anneal the carbon layer before depositing the top electrode, eventually, under a UV lamp and in N2, Ar, H2, CO, CO2, He or Xe;        dope carbon with one or more of: H, B, N, Si, Ti and another compound between SiO2, SiON, Si3N4, C3N4, BN, AlN, Al2O3, SiC.        
In other approaches, resistive switching has also been reported in graphene oxide and graphite oxide. Graphite oxide is a compound of carbon, oxygen and hydrogen, obtained by treating graphite with strong wet oxidizers. Like graphite, graphite oxide has a layered and planar structure. Each layer consists of carbon atoms arranged in a hexagonal lattice and epoxy, hydroxyl and carboxyl groups bonded to some of the carbon atoms. Graphene oxide is the monolayer form of graphite oxide and is obtained by dispersing graphite oxide in basic solutions.